Morphology & anatomy of flowering plants — root, stem, leaf

Morphology & anatomy of flowering plants — root, stem, leaf

{{TABLE: title=The Plant's Three Main Organs: A Functional Overview

Introduction: Morphology vs. Anatomy

Welcome to the foundational study of flowering plants, or angiosperms. To truly understand these complex organisms, we study them at two levels: morphology and anatomy.

• Morphology is the study of the external form and features of a plant. When we talk about the shape of a leaf, the type of root system, or the arrangement of flowers, we are discussing morphology. It's what you can see with the naked eye.
• Anatomy, on the other hand, is the study of the internal structure and organisation of a plant. This involves looking at tissues and cells, often with the help of a microscope. Understanding how xylem and phloem are arranged in a stem is anatomy.

A typical flowering plant body consists of two main systems: the Root System (usually underground) and the Shoot System (usually aerial), which includes the stem, leaves, flowers, and fruits. In this lesson, we will focus on the vegetative parts: the root, stem, and leaf.

The Root System

The root is the non-green, underground part of the plant that develops from the elongation of the radicle of the embryo. Its primary functions are anchorage and absorption.

Root Morphology: External Features

Roots are positively geotropic (grow towards gravity) and negatively phototropic (grow away from light). They lack nodes and internodes.

{{VISUAL: diagram: Labelled diagram showing the regions of a typical root tip, from the root cap to the region of maturation.}}

Regions of the Root

A typical root tip is divided into four main regions:

1. Root Cap: A thimble-like, protective structure covering the tender apex of the root. It secretes mucilage to lubricate the soil, easing the root's passage.
2. Region of Meristematic Activity: Located just behind the root cap, this region contains small, thin-walled cells with dense protoplasm called apical meristems. These cells divide rapidly to produce new cells for growth.
3. Region of Elongation: Proximal to the meristematic region, the cells here undergo rapid elongation and enlargement. This is the region primarily responsible for the growth of the root in length.
4. Region of Maturation: This is where the elongated cells differentiate and mature into specialised tissues. The most distinctive feature of this region is the presence of very fine, delicate, thread-like structures called root hairs, which are extensions of the epidermal cells and vastly increase the surface area for water absorption.

Types of Root Systems

There are three main types of root systems found in flowering plants:

• Tap Root System: The primary root, which originates from the radicle, grows deep into the soil and gives rise to lateral roots of several orders (secondary, tertiary). This is characteristic of dicotyledonous plants like mustard, gram, and mango.
• Fibrous Root System: In these plants, the primary root is short-lived and is replaced by a large number of thin, fibre-like roots originating from the base of the stem. This is characteristic of monocotyledonous plants like wheat, rice, and grass.
• Adventitious Roots: These are roots that arise from any part of the plant other than the radicle. They can grow from nodes, internodes, or leaves. Examples include the prop roots of banyan and the stilt roots of maize.

{{KEY: type=points | title=Key Root Modifications | text=

• Storage: Roots are modified to store food. Examples: Conical (carrot), Fusiform (radish), Napiform (turnip), Tuberous (sweet potato).
• Support: Roots provide extra support to the plant. Examples: Prop roots of Banyan, Stilt roots of Maize and Sugarcane.
• Respiration: In swampy areas (mangroves), roots grow vertically upwards, out of the water-logged soil, to get oxygen. These are called pneumatophores. Example: Rhizophora. }}

Root Anatomy: Internal Structure

Internally, the arrangement of tissues in roots is distinct. Let's look at a transverse section (T.S.) of a young dicot root.

The tissue layers from the outside in are:

1. Epiblema (Epidermis): The outermost single layer. Some cells extend to form unicellular root hairs. Cuticle and stomata are absent.
2. Cortex: A large zone of thin-walled parenchyma cells with intercellular spaces. It is responsible for the transport of water from the root hairs to the xylem and for food storage.
3. Endodermis: The innermost layer of the cortex. It is a single layer of barrel-shaped cells without intercellular spaces. The radial and tangential walls have a deposition of a waterproof, waxy material called suberin in the form of Casparian strips. These strips block the apoplastic pathway of water, forcing it to enter the xylem through the symplast.
4. Stele (Vascular Cylinder): All tissues inside the endodermis constitute the stele. This includes the pericycle, vascular bundles, and pith.
   • Pericycle: A few layers of thick-walled parenchyma cells next to the endodermis. It is the site of origin for lateral roots and vascular cambium during secondary growth.
   • Vascular Bundles: These are radial (xylem and phloem are on different radii) and exarch (protoxylem lies towards the periphery and metaxylem towards the centre). The number of xylem bundles is usually 2 to 6 (diarch to hexarch) in dicots.
   • Pith: A small, inconspicuous region of parenchyma cells at the centre. In many dicot roots, it is absent.

{{VISUAL: diagram: Transverse section of a young dicot root showing all tissue layers: epiblema with root hair, cortex, endodermis with Casparian strips, pericycle, and a tetrarch radial vascular bundle.}}

{{TABLE: title=Comparing Dicot and Monocot Root Anatomy

The Stem

The stem is the ascending part of the plant axis, developing from the plumule of the embryo. It bears leaves, flowers, and fruits.

Stem Morphology: External Features

The defining features of a stem are the presence of nodes (points where leaves arise) and internodes (the portion between two nodes). Stems bear buds, which can be apical (at the tip) or axillary (in the axils of leaves). Young stems are generally green and photosynthetic.

{{KEY: type=exam | title=Identifying Stem vs. Root | text=The most definitive morphological feature to distinguish a stem from a root is the presence of nodes and internodes. Even underground stems like potatoes have "eyes" which are nodes with axillary buds.}}

Stem Modifications

Stems are modified to perform various functions other than support and conduction.

• Underground Stems: These are modified for food storage and perennation (surviving unfavourable conditions).
  • Tuber: Swollen underground stem tip (e.g., Potato). The "eyes" are nodes.
  • Rhizome: Horizontal underground stem with nodes and internodes (e.g., Ginger, Turmeric).
  • Corm: Condensed, vertical, spherical underground stem (e.g., Colocasia, Saffron).
  • Bulb: Highly condensed stem with fleshy, scale-like leaves (e.g., Onion, Garlic).
• Sub-aerial Stems: These are weak stems that grow horizontally on the ground or just beneath it, for vegetative propagation. Examples include runners (grass), stolons (jasmine), and offsets (pistia).
• Aerial Stems: Modified for various special functions.
  • Stem Tendrils: Slender, spirally coiled structures for climbing (e.g., Grapevine, Cucumber).
  • Thorns: Hard, woody, pointed structures for protection (e.g., Citrus, Bougainvillea).
  • Phylloclade: Flattened or cylindrical green stems that perform photosynthesis in plants where leaves are reduced or absent (e.g., Opuntia, Euphorbia).

{{VISUAL: diagram: Examples of stem modifications - potato tuber showing 'eyes' (nodes), ginger rhizome with nodes and internodes, and tendrils of grapevine coiling for support.}}

Stem Anatomy: Internal Structure

A transverse section of a young dicot stem reveals the following structure from periphery to centre:

1. Epidermis: The outermost protective layer, covered with a waxy cuticle to prevent water loss. It may bear multicellular hairs (trichomes) and a few stomata.
2. Cortex: Differentiated into three sub-zones:
   • Hypodermis: A few layers of collenchyma cells just below the epidermis, providing mechanical strength.
   • General Cortex: Parenchymatous cells below the hypodermis.
   • Endodermis: The innermost layer of the cortex, rich in starch grains and thus also called the starch sheath.
3. Stele:
   • Pericycle: Lies on the inner side of the endodermis, present as patches of sclerenchyma (known as bundle cap).
   • Vascular Bundles: Arranged in a distinct ring. They are conjoint (xylem and phloem together), collateral (phloem on the outer side of xylem), open (with a strip of cambium between xylem and phloem), and endarch (protoxylem towards the centre).
   • Pith: A large, central region of parenchyma cells with large intercellular spaces.

{{VISUAL: diagram: Transverse section of a young dicot stem showing the ring arrangement of open, conjoint, and collateral vascular bundles, along with epidermis, cortex, and a large central pith.}}

Monocot stems differ significantly. They have a sclerenchymatous hypodermis, no distinct endodermis or pericycle, and numerous vascular bundles that are scattered in the ground tissue. The vascular bundles are conjoint, collateral, and closed (no cambium), with peripheral bundles being smaller than the central ones.

The Leaf

The leaf is a lateral, generally flattened structure borne on the stem, developing from a leaf primordium at the apical meristem. It is the primary site of photosynthesis.

Leaf Morphology: External Features

A typical leaf consists of three main parts:

1. Leaf Base: The part of the leaf attached to the stem. It may bear two small leaf-like structures called stipules. In monocots, the leaf base expands into a sheath that covers the stem.
2. Petiole: The stalk that connects the leaf blade (lamina) to the stem. Leaves without a petiole are called sessile.
3. Lamina (Leaf Blade): The green, expanded part of the leaf with veins and veinlets. The central prominent vein is the midrib.

Venation and Types of Leaves

• Venation: The arrangement of veins and veinlets in the lamina.
  • Reticulate Venation: Veinlets form an intricate network. This is characteristic of dicots.
  • Parallel Venation: Veins run parallel to each other within the lamina. This is characteristic of monocots.
• Types of Leaves:
  • Simple Leaf: The lamina is entire or, when incised, the incisions do not touch the midrib.
  • Compound Leaf: The incisions of the lamina reach up to the midrib, breaking it into a number of leaflets. These can be pinnately compound (leaflets on a common axis, the rachis, e.g., Neem) or palmately compound (leaflets attached at a common point, e.g., Silk Cotton).

{{KEY: type=definition | title=Phyllotaxy | text=Phyllotaxy is the pattern of arrangement of leaves on the stem or branch. The three main types are: Alternate (a single leaf at each node), Opposite (a pair of leaves at each node, opposite to each other), and Whorled (more than two leaves arise at a node and form a whorl).}}

Leaf Anatomy: Internal Structure

The anatomy of a dorsiventral (dicot) leaf shows three main parts: epidermis, mesophyll, and vascular system.

1. Epidermis: Covers both the upper (adaxial) and lower (abaxial) surfaces. It is covered by a cuticle. The lower epidermis generally has more stomata (pores for gas exchange) than the upper epidermis. Each stoma is guarded by two bean-shaped guard cells, which regulate its opening and closing.
2. Mesophyll: The tissue between the upper and lower epidermis, composed of parenchyma containing chloroplasts. It is differentiated into:
   • Palisade Parenchyma: Elongated cells arranged vertically and parallel to each other below the adaxial epidermis. These are the main sites of photosynthesis.
   • Spongy Parenchyma: Oval or round and loosely arranged cells below the palisade layer, with large intercellular air spaces for gaseous exchange.
3. Vascular System: Seen as veins and the midrib. The vascular bundles are conjoint and collateral, and are surrounded by a layer of thick-walled cells called the bundle sheath.

An isobilateral (monocot) leaf has stomata on both surfaces, and the mesophyll is not differentiated into palisade and spongy parenchyma.

{{VISUAL: diagram: T.S. of a dorsiventral (dicot) leaf showing upper epidermis, palisade mesophyll, spongy mesophyll, vascular bundle (xylem + phloem), lower epidermis with guarded stomata.}}

By understanding the distinct morphological and anatomical features of the root, stem, and leaf, you can not only identify a plant part but also deduce its function, its classification as a monocot or dicot, and the environmental adaptations it possesses.

{{KEY: type=points | title=Quick Revision: Dicot vs Monocot Key Differences | text=

• Roots: Dicot - Tap root system; Monocot - Fibrous root system.
• Stem: Dicot - Vascular bundles in a ring, open (cambium present); Monocot - Vascular bundles scattered, closed (no cambium).
• Leaf: Dicot - Reticulate venation, dorsiventral anatomy; Monocot - Parallel venation, isobilateral anatomy.
• Flowers: Dicot - Tetramerous or pentamerous (floral parts in multiples of 4 or 5); Monocot - Trimerous (floral parts in multiples of 3).
• Embryo: Dicot - Two cotyledons; Monocot - One cotyledon. }}